This invention relates to flow control devices, such as chokes for hydrocarbon wells.
In a hydrocarbon well, chokes control flow of fluid into production tubing from the well bore or into regions of the well bore from the production tubing. Conventionally, such chokes have been simple on/off devices that merely filly opened or fully closed the production tubing. Recently, there has been a requirement for variable flow control which has given rise to particular problems. A conventional variable flow control choke is shown in the schematic drawing of FIG. 1a. 
The basic features of this device are an outer sleeve 1 and an inner sleeve 2, each having respective sets 3, 4 of apertures located about their respective circumferences. The outer sleeve 1 may be an integral part of a section of production tubing. The inner sleeve 2 is slidably moveable by means of an actuator (not shown). FIG. 1a shows the location of the sleeves in a xe2x80x9cclosedxe2x80x9d position. FIGS. 1b and 1c show the relative positions of the sleeves in two different xe2x80x9copenxe2x80x9d positionsxe2x80x94partly open and fully open, respectively. The arrows of FIGS. 1b and 1c represent the flow of fluid from the well bore into the production tubing via the apertures 3, 4.
Annular seals 5, 6 and 7 are located between the inner 2 and outer 1 sleeves. These seals separate the annular gap between the inner and outer sleeves into chambers whilst allowing the inner sleeve to move freely. For example, there is an annular chamber 8 between seals 6 and 7, which chamber includes the apertures 3 of the outer sleeve 1.
Activation of the actuator causes the inner sleeve 2 to be moved in the direction of the arrows shown in FIG. 1a. FIG. 1b shows the apparatus of FIG. 1a in a partially open position, wherein the apertures 4 of the inner sleeve encroach on the chamber 8, thereby opening up a flow path. In the fully open position of FIG. 1c, the apertures 4 of the inner sleeve are located entirely within the chamber 8.
Various problems may be encountered with this conventional type of flow control device. For example, as the device begins to enter an xe2x80x9copenxe2x80x9d position, pressure on one side of the seal tends to distort the seal and extrude it in the direction of fluid flow. Therefore, in the example shown in FIG. 1b, the seal 6 tends to be extruded into the apertures of the inner sleeve. Should the fluid flow be in the opposite direction (i.e. from the production tubing to the well bore), the seal tends to extrude into the annular gap between the sleeves.
Another problem with this type of flow control device is that, at the point of opening, the fluid is flowing very quickly through the apertures, and at high pressure, with the result that the seal 6 can be damaged or dragged into the apertures 4 of the inner sleeve.
The high velocity of the fluid flow in the xe2x80x9cjust openxe2x80x9d position of FIG. 1b can also cause another problem, namely that of erosion of the edges of the apertures, particularly when the fluid is contaminated with solid particles such as sand. Yet another problem which may be encountered with conventional flow control devices is that the increase in fluid flow rate is not linear with linear movement of the tube and so accurate variable flow control is difficult, especially when low flow rates are required.
In accordance with a first aspect of the invention there is provided a flow control device comprising an outer sleeve having at least one aperture through its wall, an inner sleeve having at least one aperture through its wall and means for providing relative sliding movement of the sleeves between xe2x80x9copenxe2x80x9d positions allowing variable flow of fluid through the apertures of the sleeves and xe2x80x9cclosedxe2x80x9d positions, characterised by a pressure-reducing region arranged to reduce the pressure of fluid flowing through the at least one aperture of one of the sleeves.
The provision of a pressure-reducing region reduces the risk of damage of the seal 6 and reduces the likelihood of it being dragged into the apertures of the sleeve. Furthermore, erosion of the apertures is reduced.
According to a second aspect of the invention, there is provided a flow control device comprising an outer sleeve having at least one aperture through its wall, an inner sleeve having at least one aperture through its wall, means for providing relative sliding movement of the sleeves between xe2x80x9copenxe2x80x9d positions allowing variable flow of fluid through the apertures of the sleeves and xe2x80x9cclosedxe2x80x9d positions and a sealing arrangement between the inner and outer sleeves comprising at least one seal, characterised by seal bypass means arranged to permit a portion of fluid to seep around the seal so that the fluid pressure acting on a region of the seal is reduced.
The provision of a seal bypass reduces the likelihood of extrusion of the seal which was hitherto encountered. As the fluid seeps around the seal, the pressure on the opposite sides of the seal tends to equalise.
According to a further aspect of the invention, there is provided a flow control device comprising an outer sleeve having at least one aperture through its wall, an inner sleeve having at least one aperture through its wall and means for providing relative sliding movement of the sleeves between xe2x80x9copenxe2x80x9d positions allowing variable flow of fluid through the apertures of the sleeves and xe2x80x9cclosedxe2x80x9d positions, characterised in that an edge region of the at least one aperture of one of the sleeves includes erosion resistant means.
The provision of an erosion resistant means prolongs the lifetime of the flow control device. Preferably the erosion resistant means includes tungsten.
A further aspect of the invention comprises the provision of tapered edge regions for the apertures of the sleeves, also for the purpose of reducing erosion.
According to yet another aspect of the invention there is provided a flow control device comprising an outer sleeve having a set of apertures through its wall, an inner sleeve having a set of apertures through its wall and means for providing relative sliding movement of the sleeves between xe2x80x9copenxe2x80x9d positions allowing variable flow of fluid through the apertures of the sleeves and xe2x80x9cclosedxe2x80x9d positions, characterised in that one set of apertures includes an aperture extending beyond the others in the direction of opening movement.
The provision of the extended aperture enables low flow rates to be achieved when the device enters a xe2x80x9cjust openxe2x80x9d position.
Preferably the shape, size and spacing of the apertures is arranged to provide a constant percentage change of the velocity co-efficient characteristic of the fluid with linear movement of the inner sleeve.
The invention will now be described, by way of example, with reference to the accompanying drawings.